Substrate noise is one of the key problems in mixed analog/digital ICs. Although measures are known to reduce substrate noise, the noise will never be completely eliminated since this requires larger chip area or exotic packages and thus higher cost. Analog circuits on digital ICs simply have to be resistant to substrate noise. A general strategy is given which can be summarized as: the supply of the analog circuits must be referred to the substrate and the analog signals must be referred to a clean analog ground. Furthermore several design constraints are given to minimize the effect of substrate noise on analog. Two bandgap circuits are discussed and it is shown that apparently minor design issues, such as the connection of an n-well of a PMOS differential pair, can have large impact on the substrate sensitivity of this circuit. This has been verified by measurements.
|Publisher||Kluwer Academic Publishers|